Compensated resistance bridge-type electrical thermometer



J. MASSEY 3,541,857

COMPENSATED RESISTANCE BRIDGE-TYPE ELECTRICAL THERMOMETER Nov. 24, 1970.

Filed Nov. 27, 1968 Fle l TEMPERATURE.

FIG.3

INVENTOR Joan Mass-av United States Patent C 3,541,857 COMPENSATEDRESISTANCE BRIDGE-TYPE ELECTRICAL THERMOMETER John Massey, 517 EastBuilding, Hunting Towers, Mount Vernon Blvd., Alexandria, Va. 22314Filed Nov. 27, 1968, Ser. No. 779,558 Int. Cl. G01k 7/24 U.S. Cl. 73-36212 Claims ABSTRACT OF THE DISCLOSURE A resistance bridge-type electricalthermometer including a temperature-sensing resistor having a nonlineartemperature resistance characteristic in one arm of the bridge, and acompensating resistor made of the same material as thetemperature-sensing resistor in another arm of the bridge. Both thetemperature-sensing resistor and the compensating resistor are containedin a single sensor probe, so that two resistors are always exposed tothe same temperature. The compensating resistor is connected in thebridge in such a manner that it compensates for the nonlineartemperature-resistance characteristic of the temperature-sensingresistor so that the change of null adjustment of the bridge for a giventemperature change remains substantially constant over a preselectedtemperature range.

The present invention relates generally to bridge-type electricalthermometers and, more particularly, to an improved resistancebridge-type thermometer which provides a linear relationship between thesensor temperature and null adjustment.

Heretofore, it has been recognized that the null adjustment of manybridge-type thermometers is a nonlinear function of the sensedtemperature because the resistance change for each degree of temperaturechange varies over the range of temperature being sensed, i.e., theresistance-temperature coefiicient is not linear. A number of differentsolutions have been proposed to compensate for this nonlinearity, butall the solutions suggested thus far have had certain technical and/oreconomic shortcomings.

It is, therefore, a primary object of the present invention to providean improved resistance bridge-type thermometer which provides a highlyaccurate and reliable output by means of an improved and yet economicalcircuit arrangement.

It is another object of the present invention to provide an improvedbridge-type electrical thermometer of the foregoing type which providesa substantially linear resistance-temperature coeflicient over arelatively wide temperature range.

Still another object of the invention is to provide such an improvedbridge-type thermometer which facilitates optimization of the bridgeparameters to minimize the deviations from ideal linearity and therebyprovide relatively precise temperature measurements.

A further object of the invention is to provide an improved bridge-typeelectrical thermometer of the type described above which is readilyadaptable to control applications as well as temperature sensingapplications.

Yet another object of the invention is to provide such an improvedbridge-type electrical thermometer which can be efficiently manufacturedat a relatively low cost.

Other objects and advantages of the invention will become apparent fromthe following detailed description and the accompanying drawings, inwhich:

FIG. 1 is a schematic circuit diagram of a bridge circuit for use in anelectrical thermometer embodying the present invention and using atemperature-sensing element made of nickel;

FIG. 2 is a graph showing the variations in the nulling adjustments ofthe variable resistance in the bridge circuit of FIG. 1 as a function oftemperature; and

FIG. 3 is a schematic circuit diagram of a modified bridge circuit foruse in an electrical thermometer embodying the invention and using atemperature-sensing element made of platinum.

While the invention is susceptible of various modificatics andalternative forms, certain specific embodiments thereof have been shownby way of example in the drawings which will be described in detailherein. It should be understood, however, that it is not intended tolimit the invention to the particular forms disclosed but, on thecontrary, the intention is to cover all modifications, equivalents, andalternatives falling within the spirit and scope of the invention asexpressed in the appended claims.

Turning now to the drawings, FIG. 1 illustrates an electrical resistancebridge in which three of the four bridge arms contain fixed resistorsR1, R2, and R3, respectively. The fourth arm of the bridge comprises atemperature responsive sensing resistor Rs which is contained within asensor probe indicated schematically at 10, with the sensing resistor Rsbeing connected to the main portion of the bridge circuit by means of aplurality of leads 11, 12, 13 and 14. As will be apparent to thosefamiliar with the art of electrical bridges, the illustrative bridge isbalanced or at null when the cross products of the bridge resistancesare equal, i.e., when the product of the resistances of the armscontaining R1 and R3 is equal to the product of the resistances of thearms containing R2 and Rs. To determine When the bridge is balanced, anA-C power supply such as a 1000-c.p.s. oscillator 15 is connected acrossthe bridge in one direction, and a null detector such as a synchronousdemodulator 16 is connected across the bridge in the other direction.The bridge is initially adjusted to be in balance, and when it issubsequently unbalanced by a change in the resistance of Rs due to achange in the sensed temperature, a linear variable resistor Rvconnected in series with resistor R3 is adjusted until the null detector16 indicates that the bridge has been re stored to balance. By recordingthe value of resistor Rv required to balance or null the bridge at anumber of sensor temperatures, during calibration, the sensortemperature can be accurately determined (in use) by consulting therecorded data.

It will be understood that the linear variable resistor Rv may becontinuously variable, as in the case of a potentiometer, or variable inincrements, as in the case of a decade resistance box for example.

As mentioned previously, one of the problems that has been encounteredwith electrical thermometers of the type described thus far has been thenonlinear nature of the resistance-temperature coefficient thereof. Thatis, the slope of the Rv/T curve is not constant over the entiretemperature range of interest. Consequently, a given adjustment of Rv torestore the bridge to balance cannot be used to indicate temperaturedirectly, and thus it is necessary to correlate each nulling adjustmentof Rv with previously prepared tables, charts or the like in order toaccurately determine the temperature. Many other proposals have beenmade for compensating for the nonlinearity of the resistancetemperaturecoefficient automatically, but all of these proposals have certainteuhnical and/ or economic shortcomings.

In order to illustrate the non-linear nature of the bridge arrangementdescribed thus far, i.e., the bridge arrangement of FIG. 1 without theresistor Re to be dis- 3 cussed below, assume that the tempertaure rangeof interest is from -40 F. (t to 260 F. (t and R2 is 5900 ohms. At null:

at Rv =0, Rs R2=R1R3 sensing resistor R8 and has a similartemperature-resistance characteristic, the two resistances Rs and Realways vary in direct proportion to each other.

One of the significant advantages of the present invention is that thevalues of the various resistors included in R RV at tzRvz 3000 ohmsRgzRz RM 3+ 2) the bridge c1rcu1t can be readlly opt1m1zed to mlnimizeR3 errors due to deviations from a perfect linear relationship R82R3-l-Rv between the nulling adjustments in Rv and the corresolving forR3 and R1; sponding temperature changes. In this connection, it has 10been determined that the nulling value of the adjust- 1) 1 2 Ohms ableresistor Rv in the bridge arrangement of FIG. 1

Rev-R8 varies with temperature according to the equation rep- 2 S Rsresented by the curve A shown in FIG. 2. Thus it can Rl= -=358.488 ohmsb h RUARSI) e seen L at t ere are mlnor deviatlons from the perfectlylinear relationship represented by the broken line Using the abovevalues, the resistance values in the B 1n FIG. 2. With the presentinvention, it has been found bridge circuit of FIG. 1 without resistorRc at 30 F. that the deviations from erfect linearity can beminitemperature intervals over the range of =40 F. to 260 mlzed to suchan extent that the linearity of the output F. are as follows:characteristic is improved by two orders of magnitude. For example,using the same Rv value (3000 ohms) and Error. R temperature range (-40F. to 260 F.) mentioned predeviatipn viously, and assuming that point Min FIG. 3 is midway Rs RV+R3 Increment Immhnmmy between points 0 and T,the fixed resistors can be calr) n 9 "58 0 0000 culatecl to have thevalues R1=147.263l ohms, 315 353 122 4, n R2=5878.3 ohms, andR3=6860.390 ohms by the 40L 2 259-428 lowing equations (Rs being nickelwire): 290.124 4, 761. 584 269. 6%; L2

.200 5 041.839 280.2 go? 020 5834805 292.466 -14.85 At pomt 0 RS132: D343. 560 5 638. 588 4% Ru 329.394 9.82 At P0111t 1 l -P 3+ W 403.770 6,626. 763 -g gg R 425.460 6,982.75 5. s

Where Kl: =1.33934 R81) In accordance with the present invention, acompen- A422 sating means including a second temperature-sensing K s 2:means is included in the sensor probe for producing a At pom]D 2R R 2 13R0) second nonlinear electrical characteristic representing the WhereE& 1.75324 sensed temperature w1th the who of the second character- Ristic to the characteristlc of the primary temperature: RlzRc MK2+ K1sensing means being a constant, and means are opera tively connected tothe two temperature-sensing means to R R P multiply or divide the firstcharacteristic by the second Ll i characteristic to produce a linearelectrical characteristic R8 representing the sensed temperature. Thus,in the illustra- R R K1 tive embodiment of FIG. 1, a compensatmgresistor hav- 5 y 1)K1K2 K1 ing a nonlinear temperature-res1stancecharacterisnc similar to that of the temperature-sensing resistor iscon- Using the above values, the resistance values in the tained in thesensor probe along with the temperaturebridge circuit of FIG. 1 at 30 F.temperature intervals Sensing resistor, and this compensating resistoris conover the range of 40 F. to 260 F. are as follows:

Error deviation, Rs R2 R0 Ito-H21 Rs R2 R3+Rv Increment F.

nected in series with one of the fixed resistors in the As can be seenfrom the above table, the linearity of bridge to compensate for thenonlinear temperature-re- 5 the temperature-resistance characteristic isimproved by sistance characteristic of the temperature-sensing resistortwo orders of magnitude over the uncompensated bridge so that thenulling adjustment of the bridge for a given arrangement describedpreviously. temperature change remains substantially constant over Thebridge circuit of FIG. 1 is, of course, designed for the entirepreselected temperature range. More particuuse with atemperature-sensing resistor made of a materilarly, a compensatingresistor Rc made of the same maal such as nickel which has an upwardlysloping temperaterial as the sensing resistor Rs, is contained in thesensor tune-resistance characteristic, i.e., the resistance of the probe10 and connected in series with the resistor R1 so sensing resistorchanges at an increasing rate with increasthat it forms a part of thebridge arm opposite the arm ing temperature. Accordingly, thecompensating resistor containing the variable resistor Rv. Since thecompensat- R0 in FIG. 1 is connected in series with the fixed resistoring re istor Re is contained in the same probe as the R1 in the bridgearm adjacent the temperature-sensing re- 5 sistor Rs and opposite thearm containing the variable resistor Rv so that the cross product(Rv-l-R3) (Rc-l-Rl) is varied automatically by the effect of R inresponse to temperature changes to compensate for the nonlinearvariations in the resistance of Rs.

6 Using the above equations, a computer can be used to determine a valueof L to minimize In FIG. 3, there is illustrated a modified bridgecircuit and then determine the values of 3 and 1 to minimize for usewith a sensing resistor made of a material such as thetemperature-measuring errors due t0 deviations from platinum which has adownwardly sloping characteristic, linearity. Thus, in one example ofthe invention the above ie, having a resistance which changes at adecreasing rate equations were used to determine the following parameterwith increasing temperature. Thu in FIG, 3 the cornvalues andtemperature measurements over the range of pensating resistor R0 isconnected in series with the fixed 40 C. to +260 C, (Algebraic fitting,using equations resistor R2 in the bridge arm opposite the armcontaining similar to those described previously, was used to deterthetemperature-sensing resistor Rs so that the cross prodmine values of R:2249.83879, R =2006.60419, and net (RZ-i-Rc) (Rs) is variedautomatically by the efiect R =9766.04232.):

Error using Error using algebraic computerized Indicated fitting,fitting, R5 and Re Rc-I-R2 (Rc+R2) Rs Rv-l-R3 Temp. 0. degrees C.degrees 0.

Temperature:

33. 06830 2, 333. 807098 195, 965. 814547 2, 006. 50419 -40. 000 0.000004348 2, 339. 842517 210, 594. 528404 2,155.39532 -24.9702 -0. 020801536 2, 345. 849357 225.225.305095 2, 306. 21s84 -9. 0615 -0.0385 009112,351.829108 239,863.779792 2, 456. 10014 +4. 9496 -0. 0504 02725 2,357.78%58 254,507. 668000 2, 606. 04715 +19. 0443 -0. 0557 03830 2, 363.709478 269, 157. 205582 2, 756. 05200 +34.9448 -0. 0552 -.04309 2, 359.610148 283,811.406400 2,906.10461 49.9500 -0. 0500 04261 2, 375. 484478298,469. 362568 3,056.19567 64. 0591 -0. 0408 03781 2, 381. 332458313,130.120579 a, 206. 31547 79. 0711 -0. 0289 .02964 2, 387. 154108327. 792. 806358 3,356.45490 94.9851 -0.0149 -.01007 2 392,949408342,456.449478 3,506. @0419 +110. 0000 -0. 0000 -.0070s 398.718358357,120.133703 3, 656. 75300 125.0150 +0. 0150 00552 404. 460978 371,782. 998143 3,806. 89521 140. 0291 +0. 0291 01766 410.177248 386 444.084170 3,057. 01332 155.0414 +0. 0414 02843 415. 867178 401,102.5138594,107.11423 170.0510 +0. 0510 +.03692 421. 530768 415, 757. 387974 4,257. 17373 185.0570 +0. 0570 +.04219 427.168008 430,407. 785396 4, 407.18738 200.0583 +0. 0583 +.0433s 432. 778908 445, 052. 841035 4,557. 034215.0542 +0. 0542 +.03040 438. 363468 459,6s1.60s145 4, 707. 04153230.0437 +0. 0437 02051 443. 921688 474,323.384141 4, 856. 86390 245.0250 +0. 0200 01274 449.453558 488, 947. 084111 5, 000. 60419 +260. 00000.0000 01182 of R0 in response to changing temperature to compensate Itshould be noted that computerized fitting provides a for the non-linearvariations in the resistance of Rs. In the slightly better fit thanalgebraic fitting. circuit of FIG. 3, a D-C power source 25 is connectedto As can be seen from the foregoing data, this invention the bridge,and the null condition of the bridge is detected provides an improvedresistance bridge-type thermometer; by an electronic galvanometer 26.which produces a substantially linear output over a rela- In order toachieve an Rv-temperature characteristic tively wide temperature range,and yet the circuit is ex which conforms as closely as possible to theideal linear tremely economical. The improved bridge arrangementcharacteristic, it is preferred to determine the optimum provided by theinvention facilitates optimization of the values of the bridgeresistances by computer techniques. bridge parameters to minimize thedeviations from ideal For this purpose, the following equations may bederived t5 linearity and thereby provide precise temperature measfromthe bridge arrangement shown in FIG. 3: urements. Moreover, theresistance bridge-type thermometer of the invention is highly accurateand reliable over Rs (1) Rv+R3= (Rc+R2) long operating periods, and 1t1s read1ly adaptable to con- 1 trol applications as well astemperature-sensing applica- (2) Rs=Rs f(t)Rc=Rc f (t) tions.

R80 2 I claim as my invention: R1 w f) Raj-(m 1. A resistancebridge-type electrical thermometer com- R prising a resistance bridgehaving three arms with fixed 4) R3=i 4+ B RC0 mz resistors therein and afourth arm including a tempera- Rl +R2 ture-sensing resistor having anon-linear temperature-re- RS f sistance characteristic, saidtemperature-sensing resistor (5) Let: Rv+R3= (A+B T) being made of amaterial having a resistance which changes at an increasing rate withincreasing temperature, RS0 at least one linear variable resistorconnected in one of (6) Then error (ohms) [Rc f (0 the bridge armsadjacent to the arm containing said temperature resistor for making nulladjustments in said f l bridge, a sensor probe containing saidtemperature-sens- (7) Error ([C'.) rf 3 zf (15) mg resistor, acompensating resistor also contained in said R6 R2 A sensor probe andhaving a nonlinear temperature re- K T where K K2=?1 K sistancecharacteristic similar to that of said temperaturesensing resistor, saidcompensating resistor being con- (8) Let L= nected in series with thefixed resistor in the bridge arm R0,, opposlte the arm containing saidvariable linear resistor 9) Error 1a KIU 02+ Lf T and ad acent the armcontaining said temperature-sensing Rs A resistor, to compensate for thenonlinear temperature (10) F b R R =K S characteristic of saidtemperature-sensing resistor so that l the slope of the null adjustmentchange of said bridge for Rs 4R7) a glven temperature change remainssubstantially con- (11) 5: 3 stant over a preselected temperature range.

1 75 2. A reslstance bridge-type electrical thermometer as set forth inclaim 1 wherein said bridge includes a null detector.

3. A resistance bridge-type electrical thermometer as set forth in claim2 wherein the power source for said bridge is a source of AC power andsaid null detector is a synchronous demodulator.

4. A resistance bridge-type electrical thermometer as set forth in claim2 wherein the power source for said bridge is a source of D-C power andsaid null detector is an electronic galvanometer.

5. A resistance bridge-type electrical thermometer as set forth in claim1 wherein said temperature-sensing resistor and said compensatingresistor are made of the same material.

6. An electrical thermometer comprising the combination oftemperature-sensing means for producing a first nonlinear electricalcharacteristic representing the sensed temperature, compensating meansincluding second temperature-sensing means for producing a secondnonlinear electrical characteristic representing the sensed temperature,With the ratio of said first characteristic to said secondcharacteristic being a constant, and output means respOnsiVe to saidfirst and second sensing means for producing a linear electrical outputcharacteristic representing said sensed temperature, said output meansincluding means for multiplying or dividing said first characteristic bysaid second characteristic to produce said linear electrical outputcharacteristic.

7. An electrical instrument for producing a linear electricalcharacteristic representing a variable condition such as temperature orthe like, said instrument comprising the combination of first sensingmeans responsive to the variable condition for producing a firstnonlinear electrical characteristic representing the variable condition,second sensing means responsive to the variable condition for producinga second nonlinear electrical characteristic representing the variablecondition with the ratio of said first characteristic to said secondcharacteristic being a constant, and output means responsive to saidfirst and second sensing means for producing a linear electrical outputcharacteristic representing said variable condition, said output meansincluding means for multiplying or dividing said first characteristic bysaid second characteristic to produce said linear electrical outputcharacteristic.

8. A resistance bridge-type electrical thermometer comprising aresistance bridge having three arms with fixed resistors therein and afourth arm including a temperature-sensing resistor having a nonlineartemperature-resistance characteristic, said temperature-sensing resistorbeing made of a material having a resistance which changes at adecreasing rate with increasing temperature, at least one linearvariable resistor connected in one of the bridge arms adjacent to thearm containing said temperature-sensing resistor for making nulladjustments in said bridge, a sensor probe containing saidtemperaturesensing resistor, a compensating resistor also contained insaid sensor probe and having a nonlinear temperatureresistancecharacteristic similar to that of said temperature-sensing resistor,said compensating resistor being connected in series with the fixedresistor in the bridge arm opposite the arm containing saidtemperature-sensing resistor and adjacent the arm containing saidvariable linear resistor to compensate for the nonlineartemperature-resistance characteristic of said temperature-sensingresistor so that the slope of the null adjustment change of said bridgefor a given temperature change remains substantially constant over apreselected temperature range.

9. A resistance bridge-type electrical thermometer as set forth in claim8 wherein said bridge includes a null detector.

10. A resistance bridge-type electrical thermometer as set forth inclaim 9 wherein the power source for said bridge is a source of A-Cpower and said null detector is a synchronous demodulator.

11. A resistance bridge-type electrical thermometer as set forth inclaim 9 wherein the power source for said bridge is a source of D-Cpower and said null detector is an electronic galvanometer.

12. A resistance bridge-type electrical thermometer as set forth inclaim 8 wherein said temperature-sensing resistor and said compensatingresistor are made of the same material.

References Cited UNITED STATES PATENTS 2,764,731 9/1956 Koerner 323-692,891,218 6/1959 Werts. 3,052,124 9/ 1962 Averitt. 3,067,613 12/ 1962Rasmussen et al.

LOUIS R. PRINCE, Primary Examiner F. SHOON, Assistant Examiner U.S. Cl.X.R.

